Cavitation, the formation of the blastocoele, is the culmination of preimplantation development. It precedes, and is undoubtedly necessary for, implantation. The outer epithelial layer of the embryo, the trophectoderm, is responsible for fluid transport leading to blastocoele expansion; it is also the part of the embryo which attaches to the uterus, initiating implantation. Therefore, in order to understand how implantation is initiated, it is essential to know more about the differentiation of trophectoderm - knowledge that will be valuable in the quest to understand and control human reproduction. The goal of the proposed experiments is to discover the factors controlling trophectoderm differentiation, focusing on expression of the gene encoding the catalytic subunit of Na/K-ATPase. This enzyme is a plasma membrane sodium pump. It plays an active role in cavitation, and is most highly expressed in trophectoderm. The experiments will employ a recombinant DNA probe to identify and quantify the messenger RNA for this enzyme. The specific aims are: (1) to determine the time of appearance and kinetics of accumulation of the mRNA; (2) to ascertain whether the mRNA is subject to regulation at the translational level; (3) to determine the cell-type distribution of the mRNA; and (4) to explore the possible role of factors such as cell contact, cell surface glycoconjugates, cytokinesis, mitosis, DNA replication, and the nucleocytoplasmic ratio in the timing of Na/K-ATPase gene expression. These experiments should provide a fairly complete picture of the regulation of trophectoderm differentiation. In addition, experiments are planned to investigate the possibility that Ca-ATPase, a plasma membrane calcium pump, is also involved in cavitation. Similar experiments will be performed to study the regulation of its mRNA when cDNA probes become available.